# TopK Processor Plugin

The TopK processor plugin is a filter designed to get the top series over a
period of time. It can be tweaked to calculate the top metrics via different
aggregation functions.

This processor goes through these steps when processing a batch of metrics:

1. Groups measurements in buckets based on their tags and name
2. Every N seconds, for each bucket, for each selected field: aggregate all the measurements using a given aggregation function (min, sum, mean, etc) and the field.
3. For each computed aggregation: order the buckets by the aggregation, then returns all measurements in the top `K` buckets

Notes:

* The deduplicates metrics
* The name of the measurement is always used when grouping it
* Depending on the amount of metrics on each  bucket, more than `K` series may be returned
* If a measurement does not have one of the selected fields, it is dropped from the aggregation

## Configuration

```toml @sample.conf
# Print all metrics that pass through this filter.
[[processors.topk]]
  ## How many seconds between aggregations
  # period = 10

  ## How many top buckets to return per field
  ## Every field specified to aggregate over will return k number of results.
  ## For example, 1 field with k of 10 will return 10 buckets. While 2 fields
  ## with k of 3 will return 6 buckets.
  # k = 10

  ## Over which tags should the aggregation be done. Globs can be specified, in
  ## which case any tag matching the glob will aggregated over. If set to an
  ## empty list is no aggregation over tags is done
  # group_by = ['*']

  ## The field(s) to aggregate
  ## Each field defined is used to create an independent aggregation. Each
  ## aggregation will return k buckets. If a metric does not have a defined
  ## field the metric will be dropped from the aggregation. Considering using
  ## the defaults processor plugin to ensure fields are set if required.
  # fields = ["value"]

  ## What aggregation function to use. Options: sum, mean, min, max
  # aggregation = "mean"

  ## Instead of the top k largest metrics, return the bottom k lowest metrics
  # bottomk = false

  ## The plugin assigns each metric a GroupBy tag generated from its name and
  ## tags. If this setting is different than "" the plugin will add a
  ## tag (which name will be the value of this setting) to each metric with
  ## the value of the calculated GroupBy tag. Useful for debugging
  # add_groupby_tag = ""

  ## These settings provide a way to know the position of each metric in
  ## the top k. The 'add_rank_field' setting allows to specify for which
  ## fields the position is required. If the list is non empty, then a field
  ## will be added to each and every metric for each string present in this
  ## setting. This field will contain the ranking of the group that
  ## the metric belonged to when aggregated over that field.
  ## The name of the field will be set to the name of the aggregation field,
  ## suffixed with the string '_topk_rank'
  # add_rank_fields = []

  ## These settings provide a way to know what values the plugin is generating
  ## when aggregating metrics. The 'add_aggregate_field' setting allows to
  ## specify for which fields the final aggregation value is required. If the
  ## list is non empty, then a field will be added to each every metric for
  ## each field present in this setting. This field will contain
  ## the computed aggregation for the group that the metric belonged to when
  ## aggregated over that field.
  ## The name of the field will be set to the name of the aggregation field,
  ## suffixed with the string '_topk_aggregate'
  # add_aggregate_fields = []
```

### Tags

This processor does not add tags by default. But the setting `add_groupby_tag`
will add a tag if set to anything other than ""

### Fields

This processor does not add fields by default. But the settings
`add_rank_fields` and `add_aggregation_fields` will add one or several fields if
set to anything other than ""

### Example

Below is an example configuration:

```toml
[[processors.topk]]
  period = 20
  k = 3
  group_by = ["pid"]
  fields = ["cpu_usage"]
```

Output difference with topk:

```diff
< procstat,pid=2088,process_name=Xorg cpu_usage=7.296576662282613 1546473820000000000
< procstat,pid=2780,process_name=ibus-engine-simple cpu_usage=0 1546473820000000000
< procstat,pid=2554,process_name=gsd-sound cpu_usage=0 1546473820000000000
< procstat,pid=3484,process_name=chrome cpu_usage=4.274300361942799 1546473820000000000
< procstat,pid=2467,process_name=gnome-shell-calendar-server cpu_usage=0 1546473820000000000
< procstat,pid=2525,process_name=gvfs-goa-volume-monitor cpu_usage=0 1546473820000000000
< procstat,pid=2888,process_name=gnome-terminal-server cpu_usage=1.0224991500287577 1546473820000000000
< procstat,pid=2454,process_name=ibus-x11 cpu_usage=0 1546473820000000000
< procstat,pid=2564,process_name=gsd-xsettings cpu_usage=0 1546473820000000000
< procstat,pid=12184,process_name=docker cpu_usage=0 1546473820000000000
< procstat,pid=2432,process_name=pulseaudio cpu_usage=9.892858669796528 1546473820000000000
---
> procstat,pid=2432,process_name=pulseaudio cpu_usage=11.486933087507786 1546474120000000000
> procstat,pid=2432,process_name=pulseaudio cpu_usage=10.056503212060552 1546474130000000000
> procstat,pid=23620,process_name=chrome cpu_usage=2.098690278123081 1546474120000000000
> procstat,pid=23620,process_name=chrome cpu_usage=17.52514619948493 1546474130000000000
> procstat,pid=2088,process_name=Xorg cpu_usage=1.6016732172309973 1546474120000000000
> procstat,pid=2088,process_name=Xorg cpu_usage=8.481040931533833 1546474130000000000
```
